In MotoGP most of the "weaker" bikes are private teams, they don't have the funds to support multiple multi-million dollar bikes sitting in the paddock. Two-stroke engines fire once every revolution, four-stroke engines fire once every other revolution. This gives two-stroke engines a significant power boost. A jet kit is simply a tuning tool that adjusts how much fuel is delivered on a carburated bike.
To add a little: 2-stroke engines can make more power out of a smaller displacement engine. That's why the 500cc 2-stroke MotoGP bike (before they all went to 4 stroke 1000's) were making ~230hp, and the newest GP bikes are only making slightly more power, out of twice the displacement! They make more torque too, and also two strokes use mixed oil and gas. They also require rebuilding more often.
A jet "kit" is different sized jets for the carbs that allow you to change them out to get less fuel (leaning it out) or more fuel (richening the mixture) for tuning purposes. I commonly change jets depending on the track I'm at and weather conditions, among other things.